Distilled can actually be lethal for betta. It sucks minerals right out their body. R/O and rain water can do the same so mineral must be added. It is too pure. Tap water is usually really good but the chlorine, choromine and metals must be taken care of before going in the tank. Seachem Prime and API Stresscoat Kordon Amquel Plus are very good water conditioners and are way cheaper in the long run and healthier for fish. You may want to do the changeover very very slowly as the difference in ph, gh and kh could be massive and sudden change could shock the betta.